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Effect of agglomeration of raw materials on heat transfer of

Date:2019-03-08 17:52     writer:admin     Views:

  

(1) the change of convection heat transfer coefficient h with flow L

  Flow L refers to the movement course of particle group after it is added to the preheater. For single-stage preheater, particle group first moves downward under the action of impulse and gravity after it is added, and then moves upward with the airflow until particle velocity (positive) reaches the maximum, so as to maintain constant speed.

  The particle velocity us is defined as negative when moving down and positive when moving up. Therefore, it is a process from negative to 0 to positive in flow L, and finally reaches a constant velocity us'.Define the airflow velocity as u.

  We know that Nu=f(Re), the convection heat transfer coefficient h=f(ur).Ur is the relative velocity of gas and particle group, so the main factor affecting the convection heat transfer coefficient is the relative velocity of gas and solid!

  It is assumed that the reversing process is completed instantaneously after the particle group enters the preheater, that is, at the initial moment after the particle group is added, ur=u-0=u;Thereafter, when the particle moves upward, ur is less than u.Finally, when the particle reaches a constant velocity, ur=u-us', and ur is the minimum.

  Therefore, when the particles experience L(0 ~ L) distance from bottom to top, ur changes from large to small, and the convection heat transfer coefficient also changes from large to small.That is to say, in the early stage of gas-solid mixing, ur is the largest and the most intense stage of heat transfer, so full attention should be paid to it!

  

(2) change of particle effective surface a with flow L

  Effective surface a is defined as the total surface area of particles in a unit volume pipe.In other words, the ratio of the number of particle groups multiplied by the surface area of individual particles to the volume of the pipe through which the particles move in a fixed period of time (e.g. 1s).That is:

  A = total surface area of particle group/volume experienced by particles in the pipe

  The specific calculation formula is not listed here, but when the pipe diameter is fixed, the effective surface of the particles a very important influencing factors include: (1) mass flow of particle group, that is, the number of particles;(2) the specific surface area of a single particle or its particle size;(3) the particle motion speed, that is, the larger the motion speed, the larger the volume experienced by the particle motion in unit time, the smaller a.

  The variation of a with particle velocity is discussed here.Starting from the particle group added to the preheater, the particle group movement velocity from 0 to us'(ignoring the reversing process), that is to say, a is gradually smaller!

  The changing trend of convection heat transfer coefficient h and particle effective surface a with flow L is shown below.It can be seen that both h and a reach their maximum value when the particle group is just added to the preheater, and it gradually decreases with the increase of L. Therefore, it is necessary to pay attention to the heat transfer process after the particle group is just added!

  (3) for the particle group with particle diameter dp<100 m

  When the particle size is less than 100 m, aggregation and condensation often occur. However, the relevant laws are not clear and cannot be predicted. In particular, the effective surface a is more difficult to calculate alone.Therefore, the convection heat transfer coefficient h and effective surface a can only be combined to obtain the product ha of h and a through the measured heat transfer.

  Ha is called the heat capacity coefficient of the particle group.

  Japanese scholar professor tongrong ryungsan has calculated ha theoretically, and has measured ha by drying experiment of gypsum powder.(the above two parts are also the results of professor tong rong liang SAN)

  In the theoretical calculation, the h value is approximately calculated based on the fact that the dp is very small and Nu is approximately 2.0.Assuming that the powder is sufficiently dispersed, the effective surface area a is calculated to obtain the theoretical value.The experimental values are derived from the heat exchange.The results are as follows:

  The theoretical value ha (assuming the powder fully dispersed) 18500 W/(m3 · ℃)

  Ha between 800-1000 W/(m3 · ℃)

  The difference between the two is as great as 20 times!This shows that in the suspension system, the gas-solid heat transfer capacity of the fine powder is greatly affected by the aggregation characteristics of the powder.In the research, design of this kind of heat exchange equipment, should first focus on powder dispersion, which is the main contradiction!

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